In general, a secondary cell using non-aqueous electrolyte solution comprises an anode, a cathode, and a non-aqueous electrolyte layer. In order to form a cathode, cathode slurry comprising a lithium-transition metal oxide as a cathode active material, polyvinylidene fluoride (PVdF) as a binder and N-methyl-2-pyrrolidone (NMP) as a solvent is prepared, the cathode slurry is coated on a collector made of a metal foil, and then drying, pressing and molding steps are performed. In order to form an anode, the same method as described above is performed, except that anode slurry comprising carbon or carbon composite capable of lithium ion intercalation/deintercalation as an anode active material, polyvinylidene fluoride (PVdF) as a binder and N-methyl-2-pyrrolidone (NMP) as a solvent is used.
However, when polyvinylidene fluoride (PVdF) is used as a binder, the interfacial adhesion between a collector and an electrode active material and the close adhesion among the electrode active material molecules is low, and thus the electrode active material pressed and adhered to the collector may be separated and removed from the collector, when a coated electrode is cut to conform to a desired product size, e.g., by slitting, thereby causing the decrease of a nominal voltage or the irregularity of a cell capacity.
Additionally, repeated charge/discharge cycles of a cell may cause the separation and removal of an electrode active material by the shrink and expansion of an electrode. Accordingly, as charge/discharge cycles are repeated, the removal of the electrode active material from a collector is accelerated, thereby causing the decrease of a cell capacity. Moreover, a high-voltage state such as overcharge, or a temperature increase resulting from the high-voltage state may cause the decomposition of polyvinylidene fluoride (PVdF) to generate hydrogen fluoride, and thus generated hydrogen fluoride may cause a side reaction with the active material on the surface of the collector or a trace amount of metal lithium precipitates.
In addition to the above-mentioned problems, because polyvinylidene fluoride has a high crystallization degree, it has to be added in an amount of 2.5 wt % or more based on the total weight of the anode active material in order to prevent the removal of the active material in a cutting or punching step. Such an increased proportion of the binder to the total weight of the anode active material results in the decrease of the proportion of the active material, thereby causing the decrease of a full cell capacity.
Accordingly, as a solution to solve the above-mentioned problems, an electrode using a rubber-based binder such as a styrene-butadiene rubber (SBR) has been investigated. An SBR-based binder provides the same degree of effect as PVdF, even though it is used in an amount smaller than PVdF, and is electrochemically stable. When an SBR-based binder is used, it can be dispersed in water, so that water may be used as a solvent for electrode active material slurry, and thus it is environmental-friendly.
Meanwhile, viscosity control is essential to a coating process. In a coating process, a viscosity, a solid content, a coating layer thickness, a coating speed, a solvent evaporation speed and a solvent evaporation amount are organically related with one another. Therefore, when an SBR-based binder is used, a thickener is used to control the viscosity of electrode slurry. More particularly, a cellulose-based thickener such as carboxymethyl cellulose (CMC) has been investigated. The use of a styrene-butadiene rubber (SBR) as a binder and the use of a cellulose-based polymer as a thickener help to decrease the danger of a cell explosion and to increase a cell capacity. Additionally, the use of a thickener inhibits the settling of solid contents so that constant viscosities in the upper and lower parts of a solution and a uniform dispersion state in the solution may be retained for a long time and the viscosity of slurry may be stabilized.
However, the electrode active material slurry using the aforesaid binder and thickener has difficulties in dispersion thereof due to the difference of the specific gravity of the electrode active material and those of the binder and thickener. Therefore, the viscosity becomes difficult to control and to maintain with times, and the diffusion effect of carbon black used as a conductive agent becomes poor, thus adversely affecting the mixing with the active material, so that uniform distributions of the active material and the conductive agent over the whole surfaces of the electrode may not be obtained, thereby causing coating problems.
Because CMC (carboxymethyl cellulose) has some dispersion effect, when the amount of CMC (carboxymethyl cellulose) is increased, it is possible to improve the deterioration of the diffusion effect of carbon black and to solve the dispersion-related problems. However, in this case, the weight ratio of CMC per weight of the active material is increased, and thus the weight ratio of the active material is reduced. This results in the reduction of a cell capacity and the deterioration of cell properties. Additionally, the viscosity is increased due to the increase of the amount of a thickener to such a degree that cannot be applied in a practical process, and thus water as a solvent is added to adjust the viscosity, thereby causing the reduction of solid contents.